1. Field of the Invention
The present invention relates to a silicon carbide semiconductor device, more particularly, a silicon carbide semiconductor device having a gate electrode.
2. Description of the Background Art
In recent years, in order to achieve high breakdown voltage, low loss, and utilization of semiconductor devices under a high temperature environment, silicon carbide has begun to be adopted as a material for a semiconductor device. Silicon carbide is a wide band gap semiconductor having a band gap larger than that of silicon, which has been conventionally widely used as a material for semiconductor devices. Hence, by adopting silicon carbide as a material for a semiconductor device, the semiconductor device can have a high breakdown voltage, reduced on-resistance, and the like. Further, the semiconductor device thus adopting silicon carbide as its material has characteristics less deteriorated even under a high temperature environment than those of a semiconductor device adopting silicon as its material, advantageously.
Of such semiconductor devices employing silicon carbide as their material, there are semiconductor devices which controls appearance and disappearance of an inversion layer in a channel region in accordance with a predetermined threshold voltage so as to conduct or interrupt a current. Examples of such semiconductor devices include a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and an IGBT (Insulated Gate Bipolar Transistor). For such semiconductor devices, various studies have been conducted to adjust the threshold voltage or improve channel mobility (for example, see Sei-Hyung Ryu et al., “Critical Issues for MOS Based Power Devices in 4H-SiC,” Materials Science Forum (2009), pp. 743-748).
Here, for example, in a MOSFET with an n channel, a p type body region having p type conductivity is formed. In this p type body region, a channel region is formed. By increasing the density (doping density) of a p type impurity (for example, B (boron), Al (aluminum), or the like) in the p type body region, the threshold voltage is positively shifted in value to allow the MOSFET to become substantially a normally-off type or become the normally-off type. On the other hand, a MOSFET with a p channel is contrary to the case of the n channel. Namely, by increasing the density of an n type impurity in an n type body region, the threshold voltage is negatively shifted in value to allow the MOSFET to become substantially the normally-off type or become the normally-off type.
However, when adjusting the threshold voltage in this way, channel mobility is significantly decreased, disadvantageously. This is because the dopant with such an increased doping density causes noticeable scattering of electrons. In view of this, for example, the doping density of the p type body region is set at, for example, approximately 1×1016 cm−3 to approximately 4×1016 cm−3. This makes it difficult to freely set the threshold voltage while securing sufficient channel mobility in a conventional semiconductor device. In particular, it is difficult to allow the conventional semiconductor device to become substantially the normally-off type or become the normally-off type, disadvantageously.